1. Field of the Invention
The present invention relates to medical and diagnostic imaging, in particular the evaluation of medical image data sets in relation to the variation over time of changes which have been caused by diagnostic preparations (administered substances) in a region under examination.
2. Description of the Prior Art
The meaningfulness of an examination of the propagation of a diagnostic preparation, such as a contrast agent or a radiopharmaceutical, in an organ or a region of the body of a patient, using magnetic resonance tomography or scintillography, is adversely affected by various influences. An appropriate examination is carried out by using a sequence of measurements, which supplies exposures of the region under examination for various successive times. For an assessment of possible pathological changes, the changes in image intensity effected by the diagnostic preparation must be evaluated exactly from the image data of the series of exposures produced in such a way.
In general, reliable evaluation of the image intensity changes caused by a diagnostic preparation is made more difficult by three influences.
A first influence is the deformation and displacement of organs in the body because of bodily functions, such as the beating of the heart or breathing. Therefore, the times at which an image is obtained generally are synchronized with a defining bodily function in a measuring sequence. For example, during a perfusion measurement of the heart, the ECG signal is used to synchronize the recordings with the beat cycle of the heart. As a result, each individual recording in the measuring sequence is always obtained in the same state of deformation (phase) of the heart. However, since breathing follows a different cycle from that of the heart, there is a displacement of the heart or its representation in the individual recordings. In general, this means that the same regions of an organ under examination or a bodily region under examination are depicted at different positions in the individual images of a series of exposures. The determination of the position of a body region to be examined in an image therefore cannot be transferred to other images in the series of exposures.
In order to prevent a movement of an organ, caused by breathing, making the evaluation of the image data of a series of exposures more difficult or even impossible, the series of exposures of a measuring sequence is often carried out with the patient holding his or her breath. This necessitates great stress on the patient during the corresponding measuring sequence, as a result of which in turn an untrue stressed situation of the patient can be produced.
Furthermore, the evaluation is influenced by the characteristics of the detectors used to produce the image data. In particular, changing the sensitivity of a detector with the distance and/or direction to the object to be measured makes it more difficult to compare the measured data or the image intensities provided thereby from different positions of the region under examination. For example, in magnetic resonance tomography, identical tissues exhibit different intensities depending on the distance from the detector (reception) coil. The intensity in this case decreases with increasing distance from the coil.
If, therefore, a tissue close to the detector coil is compared with a tissue which is located at some distance from the detector coil, and if both tissues contain the same concentration of a diagnostic preparation, then the preparation results in a higher image intensity in the tissue close to the coil than the tissue remote from the coil.
A correct assessment is very difficult to carry out visually for these measurement-induced signal changes, as well as because the measured signal intensities are also influenced by the magnetic characteristics of the tissue itself, in addition to the imaging characteristics of the detector coil or the measuring system.
There are similar conditions in scintillography, wherein the measured image intensities are composed of the imaging characteristics of the gamma camera and the enrichment of the radiopharmaceutical in the tissue.
Furthermore, the image intensity or the value of the image elements of a body region in the image data set of an exposure from a measuring sequence is composed of a contribution from the tissue of this body region itself and a contribution from the concentration of the diagnostic preparation in this tissue at the time of the exposure, so that it is not possible to use the image intensity to make conclusions about the concentration of the diagnostic preparation.